Double telecentric objective lens

ABSTRACT

A double telecentric objective lens includes a front group having and a rear group each having a positive refracting power as a whole. The front group includes a first unit formed of a cemented lens composed of a convex lens and a concave lens, and a second unit including a convex lens or a cemented lens composed of a convex lens and a concave lens, and a concave lens. The rear group includes a third unit including a concave lens and a cemented lens composed of a convex lens and a concave lens, and a fourth unit formed of a cemented lens composed of a convex lens and a concave lens. In the above configuration, the optical constants of the lenses are determined so as to satisfy the following conditions: 
       n   1n   −n   1p &gt;0.1  (1) 
     ν 1p −ν 1n &gt;25  (2) 
     0.3 f   F   &lt;|r   2n |&lt;0.5 f   F   (3) 
     1.4&lt;( r   2p   /r   2n )&lt;2.7  (4) 
     n 3n &lt;n 3p    (5)

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a double telecentric objectivelens.

[0003] 2. Description of the Related Art

[0004] With recent developments in image processing technology, variousimage processing and measuring apparatuses have been commercialized.Objects to be measured are diversified and complicated, and there is aneed to measure, by image processing and measuring apparatuses,relatively large and thick machine parts, cutting tools, electronicparts, and the like that have hitherto been measured by measuringprojectors.

[0005] For this reason, the emergence of a telecentric objective lenshaving a low magnification of 1× or less is required, in which the fieldof view is wide, the depth of field is large enough to measure thickmachine parts having a stepped portion, and the like, and thetelecentricity (the degree of parallelizm between the principal ray ofan off-axis beam and the optical axis) is properly corrected.

[0006] Preferable as a telecentric objective lens used for measurementis a so-called double telecentric objective optical system in whichlenses are divided into two groups, a front group and a rear group, theback focus of the front group and the front focus point of the reargroup coincide with each other, and a diaphragm is placed at theposition where the focus points coincide. This is because the imagemagnification of the double telecentric optical system is basicallydetermined only by the focal lengths of the front group and the reargroup, regardless of the position of an object.

SUMMARY OF THE INVENTION

[0007] In view of such circumstances, it is an object of the presentinvention to provide a double telecentric objective lens having amagnification of approximately 0.2× in which aberrations and thetelecentricity of the principal ray of an off-axis beam are properlycorrected, and which is suitable for use in an image processing andmeasuring apparatus.

[0008] In order to achieve the above object, according to one aspect,the present invention provides a double telecentric objective lenshaving a double telecentric optical system. The double telecentricoptical system includes a front group having a positive refracting poweras a whole, and a rear group having a positive refracting power as awhole. The rear focus of the front group and the front focus of the reargroup coincide with each other, and a diaphragm is placed at theposition where the focuses coincide. The front group includes a firstunit formed of a cemented lens composed of a convex lens and a concavelens, and having a positive refracting power as a whole, and a secondunit including a convex lens or a cemented lens composed of a convexlens and a convex lens, and a concave lens, arranged in that order fromthe object side, and having a positive refracting power as a whole. Therear group includes a third unit including a concave lens and a cementedlens composed of a convex lens and a concave lens, and having a positiverefracting power as a whole, and a fourth unit formed of a cemented lenscomposed of a convex lens and a concave lens, and having a positiverefracting power as a whole.

[0009] The double telecentric objective lens satisfies the followingconditions:

n _(1n) −n _(1p)>0.1  (1)

ν_(1p)−ν_(1n)>25  (2)

0.3f _(F) <|r _(2n)|<0.5f _(F)  (3)

1.4<(r _(2p) /r _(2n))<2.7  (4)

n_(3n)<n_(3p)  (5)

[0010] where n_(1p) and ν_(1p) respectively represent the refractiveindex and the Abbe's number of the convex lens of the first unit, n_(1n)and ν_(1n) respectively represent the refractive index and the Abbe'snumber of the concave lens of the first unit, r_(2p) represents theradius of curvature of a surface of the convex lens or the cemented lenscomposed of the convex lens and the concave lens in the second unit thatis furthermost from an object, r_(2n) represents the radius of curvatureof an object-side surface of the concave lens in the second unit, f_(F)represents the focal length of the entire front group, n_(3n) representsthe average refractive index of the concave lenses in the third unit,n_(3p) represents the refractive index of the convex lens in the thirdunit, and the refractive indices and the focal length are values for thed-line (587.56 nm).

[0011] Conditional expression (1) specifies the refractive indices ofthe lenses included in the first unit.

[0012] If conditional expression (1) is not satisfied, the difference inrefractive index between the convex lens and the concave lens decreases.Therefore, when spherical aberration and the telecentricity of theprincipal ray are corrected, the radius of curvature of the bondingsurface decreases, and a high-order aberration, such as sphericalaberration, occurs. Moreover, since the principal ray emitted from thevicinity of the object and in parallel with the optical axis greatlydeviates from the center of the telecentric diaphragm, the principal rayof a light beam passing through the center of the telecentric diaphragmforms a large angle with respect to the optical axis, as viewed as awhole light beam.

[0013] When an object having a stepped portion is measured with such anobjective lens, a serious measurement error occurs around the object.

[0014] Conditional expression (2) specifies the Abbe's numbers of thelenses used in the first unit.

[0015] If chromatic aberration is corrected using glass materials thatdo not satisfy conditional expression (2), the refractive powers of boththe convex lens and the concave lens must be increased, and this resultsin a high-order aberration such as spherical aberration or comaticaberration. The telecentricity of an off-axis beam also deteriorates.The high-order aberration cannot be completely corrected by other lensunits.

[0016] In other words, conditional expressions (1) and (2) aredetermined so as to minimize spherical aberration, comatic aberration,and chromatic aberration, and the deterioration of telecentricity of anoff-axis principal ray.

[0017] Conditional expression (3) specifies the radius of curvature ofan object-side surface of the concave lens in the second unit.

[0018] When the radius of curvature exceeds the upper limit inconditional expression (3), spherical aberration or the like that occursat another position is not sufficiently corrected, and correction ofaberration in the entire front group is insufficient. The telecentricityof the principal ray is not also completely corrected.

[0019] When the radius of curvature falls below the lower limit inconditional expression (3), an excessive positive aberration occurs atthis surface. In this case, the balance cannot be achieved even byproducing negative aberrations at another surface, or a large high-orderaberration occurs.

[0020] Conditional expression (4) specifies the radius of curvaturer_(2p) of a surface of the convex lens or the cemented lens composed ofthe convex lens and the concave lens that is furthermost from the objectin the second unit, and the radius of curvature r_(2n) of an object-sidesurface of the concave lens in the second unit.

[0021] This condition is necessary to achieve a balance between negativeaberrations caused at the surface having the radius of curvature r_(2p)and positive aberrations caused at the surface having the radius ofcurvature r_(2n), to correct aberrations in the entire front groupincluding residual aberration caused in the first unit, and to maintainhigh telecentricity of the principal ray emitted from the object.

[0022] Positive aberrations at the surface with r_(2n) increase abovethe upper limit in conditional expression (4), and negative aberrationsat the surface with r_(2p) increase below the lower limit. When any ofthe aberrations is compensated for in another unit, the degree ofcompensation increases. Consequently, high-order aberration cannot beprevented, and the telecentricity of the principal ray of an off-axisbeam deteriorates.

[0023] That is, the conditions in conditional expressions (3) and (4)are necessary to maintain the balance of the aberrations in the entirefront group, and to properly correct the telecentricity of the principalray of the off-axis beam.

[0024] Conditional expression (5) specifies the refractive indices ofthe convex lens and the concave lenses in the third unit.

[0025] In order to minimize aberrations in the third unit includinghigh-order aberrations, such as spherical aberration and comaticaberration, the convex lens is made of a glass material having a highrefractive index. Further, in order to limit the Petzval sum so as toproperly correct astigmatism, the convex lens is made of a glassmaterial having a high refractive index, and the concave lenses are madeof a glass material having a low refractive index.

[0026] When the convex lens and the concave lenses are made of glassmaterials that do not satisfy conditional expression (5), the Petzvalsum increases, and therefore, astigmatism increases.

[0027] This condition is necessary, in particular, to properly correctastigmatism in the entire optical system including the front group andthe rear group.

[0028] Further objects, features and advantages of the present inventionwill become apparent from the following description of the preferredembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a structural view of a double telecentric objective lensaccording to a first embodiment of the present invention;

[0030]FIGS. 2A, 2B, and 2C are explanatory views showing sphericalaberration, astigmatism, and distortion in the double telecentricobjective lens of the first embodiment;

[0031]FIG. 3 is a structural view of a double telecentric objective lensaccording to a second embodiment of the present invention;

[0032]FIGS. 4A, 4B, and 4C are explanatory views showing sphericalaberration, astigmatism, and distortion in the double telecentricobjective lens of the second embodiment;

[0033]FIG. 5 is a structural view of a double telecentric objective lensaccording to a third embodiment of the present invention;

[0034]FIGS. 6A, 6B, and 6C are explanatory views showing sphericalaberration, astigmatism, and distortion in the double telecentricobjective lens of the third embodiment;

[0035]FIG. 7 is a structural view of a double telecentric objective lensaccording to a fourth embodiment of the present invention; and

[0036]FIGS. 8A, 8B, and 8C are explanatory views showing sphericalaberration, astigmatism, and distortion in the double telecentricobjective lens of the fourth embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Embodiments of the present invention will be described below withreference to the attached drawings.

[0038] [First Embodiment]

[0039]FIG. 1 shows the configuration of a double telecentric objectivelens according to a first embodiment of the present invention, and FIGS.2A, 2B, and 2C explain spherical aberration, astigmatism, and distortionin the first embodiment.

[0040] A double telecentric objective lens of this embodiment iscomposed of a front group G_(F) having a positive refracting power as awhole, and a rear group G_(R) having a positive refracting power as awhole. The front group G_(F) and the rear group G_(R) are placed so thatthe back focus of the front group G_(F) and the front focus of the reargroup G_(R) coincide with each other. A diaphragm is placed at theposition where the focuses coincide. Thus, a double telecentric opticalsystem is constructed.

[0041] The front group G_(F) comprises a first unit G₁ formed of acemented lens composed of a convex lens L₁ and a concave lens L₂, andhaving a positive refracting power as a whole, and a second unit G₂including a cemented lens composed of a convex lens L₃ and a concavelens L₄, and a concave lens L₅ arranged in that order from the objectside, and having a positive refracting power as a whole.

[0042] The rear group G_(R) includes a third unit G₃ a concave lens L₆and a cemented lens composed of a concave lens L₇ and a convex lens L₈,and having a positive refracting power as a whole, and a fourth unit G₄formed of a cemented lens composed of a concave lens L₉ and a convexlens L₁₀, and having a positive refracting power as a whole.

[0043] In such a configuration, the optical constants of the lenses areset as in Table 1 on conditions in which the object position is −150.021mm, the focal length of the front group (the focal length of the entirefront group) f_(F) is 150.02 mm, the image position is 33.170 mm, thefocal length of the rear group is 30.02 mm, and the (imaging)magnification is −0.2×. The position of the diaphragm is placed at aposition offset by 32.369 mm from a surface r₈ toward the image.

[0044] Herein, L₁ to L₁₀ denote lenses, r₁ to r₁₆ denote the radii ofcurvature of the lens surfaces, d₁ to d₁₅ denote the thicknesses of thelenses or the spaces therebetween, n₁ to n₁₀ denote the d-linerefractive indices of glass materials, and ν₁ to ν₁₀ denote the Abbe'snumbers of the glass materials. TABLE 1 r₁ 124.891 d₁ 12 n₁ 1.48749 v₁70.2 L₁ r₂ −68.541 d₂ 3 n₂ 1.62004 v₂ 36.3 L₂ r₃ −192.006 d₃ 120 r₄62.481 d₄ 7 n₃ 1.6968 v₃ 55.5 L₃ r₅ −81.461 d₅ 3 n₄ 1.64769 v₄ 33.8 L₄r₆ −112.691 d₆ 5.9 r₇ −72.237 d₇ 2.5 n₅ 1.65844 v₅ 50.9 L₅ r₈ 66.46 d₈39.604 r₉ −28.104 d₉ 1.3 n₆ 1.51742 v₆ 52.4 L₆ r₁₀ 14.468 d₁₀ 4.1 r₁₁35.034 d₁₁ 1.5 n₇ 1.59551 v₇ 39.2 L₇ r₁₂ 15.477 d₁₂ 6.3 n₈ 1.7432 v₈49.3 L₈ r₁₃ −32.728 d₁₃ 13 r₁₄ 41.016 d₁₄ 2.4 n₉ 1.6727 v₉ 32.1 L₉ r₁₅12.68 d₁₅ 8.6 n₁₀ 1.51633 v₁₀ 64.1 L₁₀ r₁₆ −30.622

[0045] Calculations are made using the above conditional expressions (1)to (5) on the basis of the above optical constants of the lenses:

n _(1n) −n _(1p)=0.13255

ν_(1p)−ν_(1n)=33.9

|r _(2n)|=0.48 f _(F)

(r _(2p) /r _(2n))=1.56

n _(3n)=1.55647<n _(3p)=1.7432

[0046] In this way, all the conditional expressions (1) to (5) aresatisfied.

[0047] That is, the following conditions are satisfied:

n _(1n) −n _(1p)>0.1  (1)

ν_(1p)−ν_(1n)>25  (2)

0.3f _(F) <|r _(2n)|<0.5f _(F)  (3)

1.4<(r _(2p) /r _(2n))<2.7  (4)

n_(3n)<n_(3p)  (5)

[0048] where n_(1p) and ν_(1p) respectively represent the refractiveindex and the Abbe's number of the convex lens of the first unit G₁,n_(1n) and ν_(1n) respectively represent the refractive index and theAbbe's number of the concave lens of the first unit G₁, r_(2p)represents the radius of curvature of a surface of the convex lens orthe cemented lens of the convex lens and the concave lens in the secondunit G₂ that at is furthermost from an object, r_(2n) represents theradius of curvature of an object-side surface of the concave lens in thesecond unit G₂, f_(F) represents the focal length of the entire frontgroup G_(F), n_(3n) represents the average refractive index of theconcave lenses in the third unit G₃, n_(3p) represents the refractiveindex of the convex lens in the third unit G₃, and the refractiveindices and the focal length are values for the d-line (587.56 nm).

[0049]FIGS. 2A, 2B, and 2C explain spherical aberration, astigmatism,and distortion in this embodiment, respectively. In these figures, d, F,and C represent the d-line, the F-line, and the C-line, NA representsthe numerical aperture on the image side, and Y′ represents the imageheight. FIGS. 2A, 2B, and 2C are obtained by tracing a light beam fromthe object toward the image, and show that the aberrations are correctedproperly.

[0050] Table 2 shows the telecentricities of the principal ray of anoff-axis beam when the object height is 10 mm, 14 mm, and 20 mm. Thesigns + and − on the angle in the telecentricity represent the directionin which the principal ray diverges with respect to the optical axis,and the direction in which the principal ray converges when theprincipal ray travels from the object toward the objective lens. Table 2also shows that the telecentricity of the principal ray of the off-axisbeam is corrected properly. TABLE 2 Object Height (Y) mm 10 14 20Telecentricity +0.0004° +0.0011° −0.0023°

[0051] [Second Embodiment]

[0052]FIG. 3 shows the configuration of a double telecentric objectivelens according to a second embodiment of the present invention, andFIGS. 4A, 4B, and 4C show spherical aberration, astigmatism, anddistortion in the second embodiment.

[0053] The double telecentric objective lens of this embodiment is alsocomposed of a front group G_(F) having a positive refracting power as awhole and a rear group G_(R) having a positive refracting power as awhole, in a manner similar to that in the first embodiment. The frontgroup G_(F) and the rear group G_(R) are placed so that the back focusof the front group G_(F) and the front focus of the rear group G_(R)coincide with each other. A diaphragm is placed at the position wherethe focuses coincide.

[0054] The front group G_(F) comprises a first unit G₁ formed of acemented lens composed of a convex lens L₁ and a concave lens L₂, andhaving a positive refracting power as a whole, and a second unit G₂including a convex lens L₃ and a concave lens L₄ arranged in that orderfrom the object side, and having a positive refracting power as a whole.

[0055] The rear group G_(R) comprises a third unit G₃ including aconcave lens L₅ and a cemented lens composed of a concave lens L₆ and aconvex lens L₇, and having a positive refracting power as a whole, and afourth unit G₄ formed of a cemented lens composed of a concave lens L₈and a convex lens L₉, and having a positive refracting power as a whole.

[0056] In such a configuration, the optical constants of the lenses areset as in Table 3 on conditions that the object position is −151.702 mm,the focal length of the front group (the focal length of the entirefront group) f_(F) is 150.06 mm, the image position is 35.445 mm, thefocal length of the rear group is 29.98 mm, and the imagingmagnification is −0.2×. The diaphragm is placed at a position offset by30.045 mm from a surface r₇ toward the image.

[0057] Herein, L₁ to L₉ denote lenses, r₁ to r₁₅ denote the radii ofcurvature of the lenses, d₁ to d₁₄ denote the thicknesses of the lensesor spaces therebetween, n₁ to n₉ denote the d-line refractive indices ofglass materials, and ν₁ to ν₉ denote the Abbe's numbers of the glassmaterials. TABLE 3 r₁ 125.36 d₁ 13 n₁ 1.48749 v₁ 70.2 L₁ r₂ −67.71 d₂4.5 n₂ 1.62004 v₂ 36.3 L₂ r₃ −187 d₃ 120 r₄ 56.56 d₄ 10 n₃ 1.57099 v₃50.8 L₃ r₅ −82.5 d₅ 6 L₄ r₆ −58.278 d₆ 3.8 n₄ 1.57501 v₄ 41.5 r₇ 66.7 d₇36.278 r₈ −40 d₈ 1.1 n₅ 1.6485 v₅ 53 r₉ 14.003 d₉ 3.5 L₆ r₁₀ 31.65 d₁₀ 2n₆ 1.58144 v₆ 40.7 r₁₁ 15.994 d₁₁ 7.3 n₇ 1.72916 v₇ 54.7 L₇ r₁₂ −26.978d₁₂ 14.2 L₈ r₁₃ 58.74 d₁₃ 2 n₈ 1.60342 v₈ 38 r₁₄ 12.496 d₁₄ 8.2 n₉1.48749 v₉ 70.2 L₉ r₁₅ −25.654

[0058] Calculations are made using the above conditional expressions (1)to (5) on the basis of the above optical constants of the lenses:

n _(1n) −n _(1p)=0.13255

ν_(1p)−ν_(1n)=33.9

|r _(2n)|=0.388 f _(F)

(r _(2p) /r _(2n))=1.42

n _(3n)=1.61497<n _(3p)=1.72916

[0059] In this way, all the conditional expressions (1) to (5) aresatisfied.

[0060]FIGS. 4A, 4B, and 4C explain spherical aberration, astigmatism,and distortion in this embodiment, respectively. In these figures, d, F,and C represent the d-line, the F-line, and the C-line, NA representsthe numerical aperture on the image side, and Y′ represents the imageheight. FIGS. 4A, 4B, and 4C are obtained by tracing a light beam fromthe object toward the image, and show that the aberrations are correctedproperly.

[0061] Table 4 shows the telecentricities of the principal ray of anoff-axis beam when the object height is 10 mm, 14 mm, and 20 mm. Thesigns + and − on the angle in the telecentricity represent the directionin which the principal ray diverges with respect to the optical axis,and the direction in which the principal ray converges when theprincipal ray travels from the object toward the objective lens. Table 4also shows that the telecentricity of the principal ray of the off-axisbeam is corrected properly. TABLE 4 Object Height (Y) mm 10 14 20Telecentricity +0.0004° +0.0011° −0.0022°

[0062] [Third Embodiment]

[0063]FIG. 5 shows the configuration of a double telecentric objectivelens according to a third embodiment of the present invention, and FIGS.6A, 6B, and 6C show spherical aberration, astigmatism, and distortion inthe third embodiment.

[0064] The double telecentric objective lens of this embodiment is alsocomposed of a front group G_(F) having a positive refracting power as awhole, and a rear group G_(R) having a positive refracting power as awhole, in a manner similar to that in the first embodiment. The frontgroup G_(F) and the rear group G_(R) are placed so that the back focusof the front group G_(F) and the front focus of the rear group G_(R)coincide with each other. A diaphragm is placed at the position wherethe focuses coincide.

[0065] The front group G_(F) comprises a first unit G₁ formed of acemented lens composed of a convex lens L₁ and a concave lens L₂, andhaving a positive refracting power as a whole, and a second unit G₂including a cemented lens composed of a convex lens L₃ and a concavelens L₄, and a concave lens L₅ arranged in that order from the objectside, and having a positive refracting power as a whole.

[0066] The rear group G_(R) comprises a third unit G₃ including aconcave lens L₆, and a cemented lens composed of a concave lens L₇ and aconvex lens L₈, and having a positive refracting power as a whole, and afourth unit G₄ formed of a cemented lens composed of a concave lens L₉and a convex lens L₁₀, and having a positive refracting power as awhole.

[0067] In such a configuration, the optical constants of the lenses areset as in Table 5 on conditions that the object position is −159.623 mm,the focal length of the front group (the focal length of the entirefront group) f_(F) is 150.00 mm, the image position is 33.175 mm, thefocal length of the rear group is 30.06 mm, and the imagingmagnification is −0.2×. The diaphragm is placed at a position offset by25.97 mm from a surface r₈ toward the image.

[0068] Herein, L₁ to L₁₀ denote lenses, r₁ to r₁₆ denote the radii ofcurvature of the lenses, d₁ to d₁₅ denote the thicknesses of the lensesor the spaces therebetween, n₁ to n₁₀ denote the d-line refractiveindices of glass materials, and ν₁ to ν₁₀ denote the Abbe's numbers ofthe glass materials. TABLE 5 r₁ 95.262 d₁ 3 n₁ 1.8061 v₁ 33.3 L₁ r₂ 51.5d₂ 12 n₂ 1.62041 v₂ 60.3 L₂ r₃ −840 d₃ 120 r₄ 87.985 d₄ 6 n₃ 1.7433 v₃49.2 L₃ r₅ −34.9 d₅ 2 n₄ 1.59551 v₄ 39.2 L₄ r₆ −137 d₆ 5 r₇ −52.483 d₇ 3n₅ 1.60738 v₅ 56.8 L₅ r₈ 88 d₈ 33.257 r₉ −27.856 d₉ 1.2 n₆ 1.51742 v₆52.2 L₆ r₁₀ 14.66 d₁₀ 4.1 r₁₁ 35.285 d₁₁ 1.5 n₇ 1.59551 v₇ 39.2 L₇ r₁₂15.64 d₁₂ 6.3 n₈ 1.7433 v₈ 49.2 L₈ r₁₃ −32.822 d₁₃ 13 r₁₄ 41.019 d₁₄ 2.5n₉ 1.6727 v₉ 32.2 L₉ r₁₅ 12.67 d₁₅ 8.6 n₁₀ 1.5168 v₁₀ 64.2 L₁₀ r₁₆ −30.5

[0069] Calculations are made using the above conditional expressions (1)to (5) on the basis of the above optical constants of the lenses:

n _(1n) −n _(1p)=0.18569

ν_(1p)−ν_(1n)=27

|r _(2n)|=0.35 f _(F)

(r _(2p) /r _(2n))=2.6

n _(3n)=1.55647<n _(3p)=1.7433

[0070] In this way, all the conditional expressions (1) to (5) aresatisfied.

[0071]FIGS. 6A, 6B, and 6C explain spherical aberration, astigmatism,and distortion in this embodiment, respectively. In these figures, d, F,and C represent the d-line, the F-line, and the C-line, NA representsthe numerical aperture on the image side, and Y′ represents the imageheight. FIGS. 6A, 6B, and 6C are obtained by tracing a light beam fromthe object toward the image, and show that the aberrations are correctedproperly.

[0072] Table 6 shows the telecentricities of the principal ray of anoff-axis beam when the object height is 10 mm, 14 mm, and 20 mm. Thesigns + and − on the angle in the telecentricity represents thedirection in which the principal ray diverges with respect to theoptical axis and the direction in which the principal ray converges whenthe principal ray travels from the object toward the objective lens.Table 6 also shows that the telecentricity of the principal ray of theoff-axis beam is corrected properly. TABLE 6 Object Height (Y) mm 10 1420 Telecentricity +0.0007° +0.0014° −0.0007°

[0073] [Fourth Embodiment]

[0074]FIG. 7 shows the configuration of a double telecentric objectivelens according to a fourth embodiment of the present invention, andFIGS. 8A, 8B, and 8C show spherical aberration, astigmatism, anddistortion in the fourth embodiment.

[0075] The double telecentric objective lens of this embodiment is alsocomposed of a front group G_(F) having a positive refracting power as awhole and a rear group G_(R) having a positive refracting power as awhole, in a manner similar to that in the first embodiment. The frontgroup G_(F) and the rear group G_(R) are placed so that the back focusof the front group G_(F) and the front focus of the rear group G_(R)coincide with each other. A diaphragm is placed at the position wherethe focuses coincide.

[0076] The front group G_(F) comprises a first unit G₁ formed of acemented lens composed of a concave lens L₁ and a convex lens L₂, andhaving a positive refracting power as a whole, and a second unit G₂including a cemented lens composed of a convex lens L₃ and a concavelens L₄, and a concave lens L₅ arranged in that order from the objectside, and having a positive refracting power as a whole.

[0077] The rear group G_(R) comprises a third unit G₃ including acemented lens composed of a concave lens L₆ and a concave lens L₇, and aconcave lens L₈ and having a positive refracting power as a whole, and afourth unit G₄ formed of a cemented lens composed of a concave lens L₉and a convex lens L₁₀, and having a positive refracting power as awhole.

[0078] In such a configuration, the optical constants of the lenses areset as in Table 7 on conditions that the object position is −161.051 mm,the focal length of the front group (the focal length of the entirefront group) f_(F) is 150.01 mm, the image position is 26.213 mm, thefocal length of the rear group is 30.01 mm, and the imagingmagnification is −0.2×. The diaphragm is placed at a position offset by26.03 mm from a surface r₈ toward the image.

[0079] Herein, L₁ to L₁₀ denote lenses, r₁ to r₁₆ denote the radii ofcurvature of the lenses, d₁ to d₁₅ denote the thicknesses of the lensesor the spaces therebetween, n₁ to n₁₀ denote the d-line refractiveindices of glass materials, and ν₁ to ν₁₀ denote the Abbe's numbers ofthe glass materials. TABLE 7 r₁ 96.68 d₁ 3 n₁ 1.8061 v₁ 33.3 L₁ r₂51.315 d₂ 12 n₂ 1.62041 v₂ 60.3 L₂ r₃ −700 d₃ 120 r₄ 88.766 d₄ 6 n₃1.7433 v₃ 49.2 L₃ r₅ −33.937 d₅ 2 n₄ 1.59551 v₄ 39.2 L₄ r₆ −134.849 d₆ 5r₇ −54 d₇ 3 n₅ 1.60738 v₅ 56.8 L₅ r₈ 81.51 d₈ 32.583 r₉ −12.159 d₉ 1.2n₆ 1.54072 v₆ 47.2 L₆ r₁₀ 10.16 d₁₀ 5 n₇ 1.741 v₇ 52.7 r₁₁ −14.72 d₁₁ 5r₁₂ −7.8 d₁₂ 3 n₈ 1.51823 v₈ 59 L₈ r₁₃ −14.259 d₁₃ 11 r₁₄ 44.09 d₁₄ 2 n₉1.7495 v₉ 35 L₉ r₁₅ 12.398 d₁₅ 6.5 n₁₀ 1.64 v₁₀ 60.2 L₁₀ r₁₆ −27.837

[0080] Calculations are made using the above conditional expressions (1)to (5) on the basis of the above optical constants of the lenses:

n _(1n) −n _(1p)=0.18569

ν_(1p)−ν_(1n)=27

|r _(2n)|=0.36 f _(F)

(r _(2p) /r _(2n))=2.5

n _(3n)=1.52948<n _(3p)=1.741

[0081] In this way, all the conditional expressions (1) to (5) aresatisfied.

[0082]FIGS. 8A, 8B, and 8C explain spherical aberration, astigmatism,and distortion in this embodiment, respectively. In these figures, d, F,and C represent the d-line, the F-line, and the C-line, NA representsthe numerical aperture on the image side, and Y′ represents the imageheight. FIGS. 8A, 8B, and 8C are obtained by tracing a light beam fromthe object toward the image, and show that the aberrations are correctedproperly.

[0083] Table 8 shows the telecentricities of the principal ray of anoff-axis beam when the object height is 10 mm, 14 mm, and 20 mm. Thesigns + and − on the angle in the telecentricity represents thedirection in which the principal ray diverges with respect to theoptical axis and the direction in which the principal ray converges whenthe principal ray travels from the object toward the objective lens.Table 8 also shows that the telecentricity of the principal ray of theoff-axis beam is corrected properly. TABLE 8 Object Height (Y) mm 10 1420 Telecentricity +0.0004° +0.0011° −0.0017°

[0084] While the focal length of the front group is approximately 150mm, the focal length of the rear group is approximately 30 mm, and themagnification is approximately 0.2× in the double telecentric objectivelenses of the above embodiments, the objective lenses can be used as adouble telecentric objective lens of 0.1×, for example, by doubling thefocal length of the front group to 300 mm and placing the rear focus ofthe front group in accordance with the telecentric diaphragm.

[0085] While the values in the above embodiments are set when the imageheight Y′ is 4 mm, the double telecentric objective lenses can be usedas a double telecentric objective lens having an image height Y′ of 8 mmby doubling the values.

[0086] It is, of course, possible to change the magnification and toincrease or decrease the effective image circle, as described above,within the technical scope of the invention.

[0087] According to the present invention, it is possible to achieve adouble telecentric objective lens having a magnification ofapproximately 0.2× in which aberrations and the telecentricity of theprincipal ray of an off-axis beam are corrected properly, and which issuitable for use in an image processing and measuring apparatus.

[0088] While the present invention has been described with reference towhat are presently considered to be the preferred embodiments, it is tobe understood that the invention is not limited to the disclosedembodiments. On the contrary, the invention is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims. The scope of the following claims is to beaccorded the broadest interpretation so as to encompass all suchmodifications and equivalent structures and functions.

What is claimed is:
 1. A double telecentric objective lens having adouble telecentric optical system, wherein said double telecentricoptical system comprises: a front group having a positive refractingpower as a whole; and a rear group having a positive refracting power asa whole, wherein the rear focus of said front group and the front focusof said rear group coincide with each other, and a diaphragm is placedat the position where the focuses coincide, wherein said front groupincludes a first unit formed of a cemented lens composed of a convexlens and a concave lens, and having a positive refracting power as awhole, and a second unit including a convex lens or a cemented lenscomposed of a convex lens and a concave lens, and a concave lens,arranged in that order from an object side, and having a positiverefracting power as a whole, wherein said rear group includes a thirdunit including a concave lens and a cemented lens composed of a convexlens and a concave lens, and having a positive refracting power as awhole, and a fourth unit formed of a cemented lens composed of a convexlens and a concave lens, and having a positive refracting power as awhole, and wherein the following conditions are satisfied: n _(1n) −n_(1p)>0.1  ( 1)ν_(1p)−ν_(1n)>25  (2)0.3f _(F) <|r _(2n)|<0.5f_(F)  (3)1.4<(r _(2p) /r _(2n))<2.7  (4)n_(3n)<n_(3p)  (5) where n_(1p)and ν_(1p) respectively represent the refractive index and the Abbe'snumber of said convex lens of said first unit, n_(1n) and ν_(1n)respectively represent the refractive index and the Abbe's number ofsaid concave lens of said first unit, r_(2p) represents the radius ofcurvature of a surface of said convex lens or said cemented lenscomposed of said convex lens and said concave lens in said second unitthat is furthermost from an object, r_(2n) represents the radius ofcurvature of an object-side surface of said concave lens in said secondunit, f_(F) represents the focal length of said entire front group,n_(3n) represents the average refractive index of said concave lenses insaid third unit, n_(3p) represents the refractive index of said convexlens in said third unit, and the refractive indices and the focal lengthare values for the d-line (587.56 nm).